Separators of this kind are used for the filtration of even weakly magnetic particles, i.e. particles of materials having a low magnetic susceptibility from a fluid, in which they are suspended, the fluid as such presenting a still lower magnetic background susceptibility. Even particles of a very small size down to colloidal or sub-colloidal size may be separated in this way. A typical large-scale industrial application is the removal of contaminants from a slurry of kaolin or China clay, but also for water purification such as the removal of ochre or other impurities and the filtration of air suspended solid particles like fly ash magnetic separation may be used.
The selective removal of particles is due to the generation of a high intensity magnetic field in the separation chamber and the presence therein of a matrix of a soft magnetic material normally in the form of steel wool, a steel wire cloth or steel balls which are magnetized and create high local magnetic field gradients, whereby the particles to be extracted are trapped by the matrix material. After a certain time of operation, the matrix will become saturated and has to be cleaned, usually by water rinsing.
In known high-gradient magnetic separators, the high intensity field considered necessary for successful operation is generated by electromagnets either of the conventional resistive coil type, or by means of superconducting electromagnetic coils, the latter of which types seems to have gained particular interest due to the very high power consumption of ordinary electromagnetic coils.
However, even if superconducting electromagnetic coils cause a very substantial reduction of the demands on electrical power, they require a cooling system to bring them into the superconducting state, whereby the construction of such separators is made complicated and expensive and is less suitable for field operation.
In addition, the generation of high intensity magnetic fields by means of electromagnetic coils whether of the conventional resistive type or of the superconducting type will normally result in limitations with respect to separator design, which counteract optimization of the filtration process.
A typical known example of a high-gradient separator is the Kolm-Marston separator disclosed in U.S. Pat. No. 3,627,678, in which the electromagnetic coil, which may be of the cryogenic or superconducting type, is arranged in a recess in a heavy iron frame providing the magnetic return path. The slurry or fluid, from which magnetizable particles are to be extracted, is made to flow through the separation chamber parallel or antiparallel to the direction of the axial magnetic field from the coil. Even if the canister containing the matrix of soft magnetic material extends substantially throughout the magnetic air gap volume limited by the coil and the adjoining yoke parts of the return frame, it has appeared that particle capture is essentially limited to the upstream side of the individual matrix members. As a result, matrix saturation will occur after a limited period of operation and frequent cleaning of the matrix will be necessary. Since cleaning requires shutdown of the magnetic field, a complex flow control system is used in the Kolm-Marston separator to allow the flow of feed slurry to by-pass the separation chamber into a fluid return circuit in the cleaning periods, so that cleaning can be performed without removing the canister from the separation chamber. Since the shutdown periods necessary for demagnetizing the matrix are relatively long the duty cycle of this prior art separator is rather low.
Some of these operational diadvantages have been remedied in a separator disclosed in U.S. Pat. No. 4,124,503 by such a design of the separation chamber that a portion of the flow path for the feed slurry extends transversely to the direction of the magnetic field. The separation chamber has the form of a cylinder surrounded by an electromagnetic coil and comprising concentrical inner and outer tubular walls. The slurry enters the chamber in the central part limited by the inner tubular wall and leaves the chamber in the peripheral part outside the outer tubular walls, whereas the matrix material is confined to the space between the inner and the outer walls in which the slurry flows radially outwards. Thus, in this design the more effective utilization of the total volume of matrix material has been achieved at the expense of a decrease in efficiency caused by the fact that a substantial part of the magnetized gap volume is not occupied by matrix material and makes, therefore, no contribution to the separation.
Another example of a separator design involving a flow path for the feed slurry directed transversely to the magnetic field direction is the separator disclosed in U.S. Pat. No. 3,819,515, in which two electomagnetic coils are arranged at each side of the separation chamber, so that the axial field produced by each coil passes through the chamber transversely to the flow direction. Thereby, the separation chamber may be completely occupied by matrix material and contrary to the separator disclosed in U.S. Pat. No. 4,124,503, the flow path may be linear throughout the chamber. A heavy iron frame providing the magnetic return path is formed with bores for slurry inlet and outlet pipes, as well as a pipe system for supplying cleaning water to the separation chamber, which is not removed during matrix cleaning. Owing to the fact that the flowpath for the cleaning agent is shorter than the flowpath for the separation process, the duty cycle will be more favourable than that of the above-mentioned Kolm-Marston separator.
In French patent No. 2,475,935, which describes a method for cleaning the filter matrix of a magnetic separator without removing it from the magnetic field by raising the temperature of the matrix above the Curie point, it has been suggested to use a permanent magnet for the generation of the magnetic field for relatively low intensity applications whereas for high-gradient applications requiring a high intensity field like those mentioned in the foregoing then use of electro magnets is prescribed.
Also in Japanese patent No. 109265/78 it has been suggested to use permanent magnets in a low-intensity separator for the collection of easily magnetizable magnetite particles from a fluid.
For high gradient low intensity separation on a laboratory scale a small size magnetic separator has also been described in an article "A Bench Top Magnetic Separator for Malarial Parasite Concentration", by F. Paul et al in IEEE, Transactions on Magnetics, VOL MAG-17, No. 6, November 1981, pages 2822 to 2824 for the extraction of red blood cells infected with malarial parasites from whole blood and involving the generation of a magnetic field in a small size filtration chamber of a volume of 2-5 cm.sup.3 by means of a conventional C-type Alnico magnet of the kind used in magnetrons.
The permanent magnet in this separator forms alone the entire magnetic circuit of the separator without such attention having been paid to the rather heavy magnetic losses in such a configuration.